Apparatus of this kind, which may briefly be referred to as "thermal surface ion sources" or "contact ionization sources" is described, for example, in the book by R. G. Wilson and G. R. Brewer "Ion Beams", New York 1973, more particularly pages 26 to 36 and pages 72 to 77. Such apparatus is based on the effect that when neutral atoms impinge upon a surface hot enough for the atoms not to be adsorbed on the surface some of the atoms are ionized on leaving the surface. Saha-Langmuir's law applies to the degree of ionization R, i.e. the ratio of the ions to the total number of particles leaving the surface, the degree of ionization R.sub.+ for positive ions being: EQU R.sub.+ =n.sub.+ /(n.sub.0 +n.sub.+)=(1+K.sub.+ exp(I-W)/kT)).sup.-1 ( 1)
and the degree of ionization R.sub.31 for negative ions being: EQU R.sub.- =n.sub.- /(n.sub.0 +n.sub.-)=(1+K.sub.- exp((W-E)/kT)).sup.-1 ( 2)
where
n.sub.+ =number of positive ions leaving the surface PA1 n.sub.- =number of negative ions leaving the surface PA1 n.sub.0 number of neutral atoms leaving the surface PA1 W=electron work function of the surface PA1 I=ionization potential of atoms PA1 E=electron affinity of atoms PA1 T=surface temperature PA1 k=Boltzmann's constant PA1 K.sub.+ and K.sub.- =statistical factors for positive and negative ions respectively (for alkaline metals K.sub.+ =2, for halogens K.sub.+ =4). PA1 V=acceleration voltage PA1 M=mass number PA1 d=distance between ionizing surface and the acceleration or extraction electrode.
If W-I&gt;0.4 eV and E-W&gt;0.4 eV, then R.sub.+ and R.sub.- have almost the same value 1, i.e. practically all the atoms impinging on the surface vaporize as positive and negative ions. For example, cesium vapour (I=3.88 eV) on impinging on a hot (1300K) tungsten surface (W=4.54 eV) is practically completely positively ionized while on the other hand, for example, iodine vapour (E=3.23 eV) impinging on a hot lanthanum hexaboride surface (W=2.70 eV) is practically completely negatively ionized. Similar high degrees of ionization can be obtained for the other alkali metals and halogens and for a number of other atoms.
It is known either to pass the particles for ionization in vapour form from the front on to the hot surface of the ionization electrode of suitable material or diffuse it from the rear through a hot frit of the appropriate material to the then porous surface. The resulting ions are then sucked away from the surface by an electric field produced between the surface and an acceleration electrode disposed at a distance in front of the same. The attainable current densities J are limited in the above cases by Child's space charge law which, for a planar arrangement is as follows: EQU J=5,45.times.10.sup.-8 V.sup.1,5 /.sqroot.M d.sup.2 A cm.sup.-2 ( 3)
where
In an apparatus known from DE-PS No. 28 05 273 C3 for the production of a beam of accelerated ions by contact ionization, the space charge limitation of the emission current density is shifted to much higher values than in the case of planar electrodes by giving the ionizing surface a highly convex curvature so that a very high electrical field strength prevails thereon. The emission current density is then limited by the vapour pressure of the element for ionization prevailing between the electrodes. This vapour pressure must be sufficiently low so that no electrical breakdown occurs due to impact ionization in the vapour.